ViewGroup的dispatchTouchEvent方法源码解读

任何一次touch事件都包含以下几个操作：

(1) ACTION_DOWN：一个touch事件首先执行的按下操作，其它的操作都是以按下操作为基础；

(2) ACTION_MOVE：一个touch事件可以有多个或者0个移动操作；

(3) ACTION_UP：一个touch事件以抬起操作为结束。

假设有一个父布局（ViewGroup），该父布局有多个子View。下面将通过这个假设来分析，每次touch事件发生时，首先调用的是ViewGroup的dispatchTouchEvent方法，这个方法内进行事件分发。

dispatchTouchEvent方法的源码：

       @Override         public boolean dispatchTouchEvent(MotionEvent ev) {             final int action = ev.getAction();             final float xf = ev.getX();             final float yf = ev.getY();             final float scrolledXFloat = xf + mScrollX;             final float scrolledYFloat = yf + mScrollY;             final Rect frame = mTempRect;             //这个值默认是false, 然后我们可以通过requestDisallowInterceptTouchEvent(boolean disallowIntercept)方法             //来改变disallowIntercept的值             boolean disallowIntercept = (mGroupFlags & FLAG_DISALLOW_INTERCEPT) != 0;             //这里是ACTION_DOWN的处理逻辑             if (action == MotionEvent.ACTION_DOWN) {              //清除mMotionTarget, 每次ACTION_DOWN都很设置mMotionTarget为null                 if (mMotionTarget != null) {                     mMotionTarget = null;                 }                 //disallowIntercept默认是false, 就看ViewGroup的onInterceptTouchEvent()方法                 if (disallowIntercept || !onInterceptTouchEvent(ev)) {                     ev.setAction(MotionEvent.ACTION_DOWN);                     final int scrolledXInt = (int) scrolledXFloat;                     final int scrolledYInt = (int) scrolledYFloat;                     final View[] children = mChildren;                     final int count = mChildrenCount;                     //遍历其子View                     for (int i = count - 1; i >= 0; i--) {                         final View child = children[i];                         //如果该子View是VISIBLE或者该子View正在执行动画, 表示该View才                         //可以接受到Touch事件                         if ((child.mViewFlags & VISIBILITY_MASK) == VISIBLE                                 || child.getAnimation() != null) {                          //获取子View的位置范围                             child.getHitRect(frame);                             //如Touch到屏幕上的点在该子View上面                             if (frame.contains(scrolledXInt, scrolledYInt)) {                                 // offset the event to the view's coordinate system                                 final float xc = scrolledXFloat - child.mLeft;                                 final float yc = scrolledYFloat - child.mTop;                                 ev.setLocation(xc, yc);                                 child.mPrivateFlags &= ~CANCEL_NEXT_UP_EVENT;                                 //调用该子View的dispatchTouchEvent()方法                                 if (child.dispatchTouchEvent(ev))  {                                     // 如果child.dispatchTouchEvent(ev)返回true表示                                  //该事件被消费了，设置mMotionTarget为该子View                                     mMotionTarget = child;                                     //直接返回true                                     return true;                                 }                                 // The event didn't get handled, try the next view.                                 // Don't reset the event's location, it's not                                 // necessary here.                             }                         }                     }                 }             }             //判断是否为ACTION_UP或者ACTION_CANCEL             boolean isUpOrCancel = (action == MotionEvent.ACTION_UP) ||                     (action == MotionEvent.ACTION_CANCEL);             if (isUpOrCancel) {                 //如果是ACTION_UP或者ACTION_CANCEL, 将disallowIntercept设置为默认的false              //假如我们调用了requestDisallowInterceptTouchEvent()方法来设置disallowIntercept为true              //当我们抬起手指或者取消Touch事件的时候要将disallowIntercept重置为false              //所以说上面的disallowIntercept默认在我们每次ACTION_DOWN的时候都是false                 mGroupFlags &= ~FLAG_DISALLOW_INTERCEPT;             }             // The event wasn't an ACTION_DOWN, dispatch it to our target if             // we have one.             final View target = mMotionTarget;             //mMotionTarget为null意味着没有找到消费Touch事件的View, 所以我们需要调用ViewGroup父类的             //dispatchTouchEvent()方法，也就是View的dispatchTouchEvent()方法             if (target == null) {                 // We don't have a target, this means we're handling the                 // event as a regular view.                 ev.setLocation(xf, yf);                 if ((mPrivateFlags & CANCEL_NEXT_UP_EVENT) != 0) {                     ev.setAction(MotionEvent.ACTION_CANCEL);                     mPrivateFlags &= ~CANCEL_NEXT_UP_EVENT;                 }                 return super.dispatchTouchEvent(ev);             }             //这个if里面的代码ACTION_DOWN不会执行，只有ACTION_MOVE             //ACTION_UP才会走到这里, 假如在ACTION_MOVE或者ACTION_UP拦截的             //Touch事件, 将ACTION_CANCEL派发给target，然后直接返回true             //表示消费了此Touch事件             if (!disallowIntercept && onInterceptTouchEvent(ev)) {                 final float xc = scrolledXFloat - (float) target.mLeft;                 final float yc = scrolledYFloat - (float) target.mTop;                 mPrivateFlags &= ~CANCEL_NEXT_UP_EVENT;                 ev.setAction(MotionEvent.ACTION_CANCEL);                 ev.setLocation(xc, yc);                 if (!target.dispatchTouchEvent(ev)) {                 }                 // clear the target                 mMotionTarget = null;                 // Don't dispatch this event to our own view, because we already                 // saw it when intercepting; we just want to give the following                 // event to the normal onTouchEvent().                 return true;             }             if (isUpOrCancel) {                 mMotionTarget = null;             }             // finally offset the event to the target's coordinate system and             // dispatch the event.             final float xc = scrolledXFloat - (float) target.mLeft;             final float yc = scrolledYFloat - (float) target.mTop;             ev.setLocation(xc, yc);             if ((target.mPrivateFlags & CANCEL_NEXT_UP_EVENT) != 0) {                 ev.setAction(MotionEvent.ACTION_CANCEL);                 target.mPrivateFlags &= ~CANCEL_NEXT_UP_EVENT;                 mMotionTarget = null;             }             //如果没有拦截ACTION_MOVE, ACTION_DOWN的话，直接将Touch事件派发给target             return target.dispatchTouchEvent(ev);         }